Most people accept that the North Sea has been subjected to the most appalling overfishing. Whitefish stocks have collapsed and the mackerel and herring fisheries are all but commercially extinct.

Under normal circumstances the removal of huge numbers of predatory fish would allow room for species such as sandeel to increase dramatically, but this is not the case.

Why is this? The sandeel fishery has now become by far the biggest single-species fishery in the North Sea, with landings accounting for one-third of all fish landed. The vast majority of this catch is landed and processed in Denmark. Such fundamental changes in the fabric of the marine ecosystem are what ecologists refer to as 'fishing down the food web'.

Since 1977, total yearly North Sea sandeel catches have fluctuated around 600,000-800,000 tonnes, but since 2003 catches have crashed dramatically to between 200,000-300,000 tonnes. The collapse of the fishery was particularly severe in the Norwegian economical zone with a 95% reduction in landings in 2005.

You will not have eaten a sandeel knowingly (unwittingly perhaps as a fish-oil supplement), so what exactly are sandeels used for? The sandeel is an exceptionally oily fish and is harvested for the rapidly expanding fish-oil and fish-meal industries and used in everything from food for farmed salmon to animal feed and health supplements. At one stage sandeels were even used to fuel Danish power stations. And demand is set to increase.

Recent forecasts by the FAO (UN Food & Agriculture Organisation) indicate that aquaculture and its insatiable appetite for fish-oil, will dominate world fish supplies by 2030, fuelling pressure for a high level of industrial sandeel fishing in the North Sea. It takes, for example, 4kg of wild caught sandeel to produce 1kg of farmed salmon.

This does not bode well for the marine species in the North Sea that depend on sandeels for food. Many scientists and ecologists believe that the recent disastrous breeding seasons for many of Europe's seabird colonies can be directly linked to the industrial fishing of sandeels in the North Sea.

As early as 1997, two respected Danish fisheries scientists - Henrik Gislason and Eskild Kirkegaard - were highly critical of the North Sea sandeel fishery, and they concluded that "it cannot be ruled out that (sandeel) fishing could adversely effect the breeding success of the birds. It would therefore be precautionary to close areas to fishing until more is known about sandeel stock structure and interactions between sandeels and seabirds". And a report published by the International Council for Exploration of the Seas (ICES) suggests that "the amount of industrial fish species taken by fishermen in the North Sea appears to leave little for seabirds and marine mammals".

It would not be unreasonable then to suggest that the overfishing of sandeel stocks may represent the single greatest threat to seabirds in the North Sea, especially in the breeding season when seabirds forage close to their colonies.

And it is not only the seabirds that are suffering. Studies into the diet of common dolphins, grey seals and harbour porpoises in Scottish waters have shown that they feed mainly on sandeels in the spring and early summer. The affects on these species if they cannot find sandeels to eat at this time of year can be disastrous. For example, spring is a critical time for dolphins and porpoises in terms of energy requirements. Some of the lowest sea temperatures occur in the North Sea in March, putting a great strain on dolphins and porpoises as they require the thickest blubber layer to limit heat loss at this time. In addition young animals are weaned and become independent foragers in spring, placing them at the mercy of changes in sandeel availability.

Ironically the long-term overfishing of sandeels in the North Sea may also inhibit a return to the former healthy status of predatory fish stocks such as cod and haddock, as these stocks can only recover if there are sufficient prey fish for them to feed upon. This is what ecologists refer to as a 'negative feedback loop', a vicious circle of exploitation that renders an ecosystem incapable of a recovery to anywhere near its former productivity.

It may not yet be too late however, but our attitudes and tolerance to an industry that has got perilously close to destroying the very fabric of the North Sea must change dramatically. No longer can we view the commercial fishing industry with the romantic notion of hard working fishermen risking everything to put fish on our plates. It must now be seen for what it is, a ruthless, efficient, hi-tech industry of destruction that is prepared to wipe out whole species for profit with almost no long-term consideration for the health of the marine environment.